Homework Help:
Relativity - running person gains mass

I do not think that any energy has been gained from 'outside' the system since the kinetic energy is derived from chemical energy that is already in the body prior to the run. So, how can the body have gained mass?

You're right. The energy was already there, albeit in different form. All that happened was a conversion from one type to another. The point they're making, as I'm sure you're aware, is that there is an inertial equivalent to energy.